In recent years, permanent magnets of high magnetic energy products have been developed based on the remarkable research and development of permanent magnets, and the downsizing and higher output of electric motors are being promoted. In particular, with an electric motor for use in vehicles such as hybrid vehicles, higher efficiency is strongly demanded for controlling the gas emission and improving the mileage. Moreover, it is demanded of higher torque and higher output within a limited space where the mounting space is small, and therefore the electric motor of higher energy density is demanded more than ever. Consequently, pursuant to the foregoing demands, the electromagnetic excitation force of the electric motors is increasing, which gives rise to problems such as the increase in vibration and noise. Particularly, quietness inside the vehicle and less noise outside the vehicle are being strictly demanded for use in hybrid vehicles.
Thus, proposed is a rotor of a reluctance-type electric motor capable of reducing torque ripples, vibrations and noise by forming the rotor laminated core in a block shape, and shifting and binding the cores in a circumferential direction so as to obtain an effect that is similar to a skew (for example, refer to Patent Document 1).
In other words, with this reluctance-type electric motor, a magnetic convex part (d-axis) where the magnetic flux can easily pass through around the rotor and a magnetic concave part (q-axis) where the magnetic flux cannot easily pass through are formed in the same number as the number of poles. This electric motor has a high void magnetic flux density in relation to the armature in the magnetic convex part, has a low void magnetic flux density in the magnetic concave part with a large magnetic resistance, and generates reluctance torque based on such changes in the magnetic flux density. Particularly, with a permanent magnet-type reluctance electric motor in which a permanent magnet is embedded in a rotor and possessing magnetic saliency, torque is generated based on the magnetic suction power and the magnetic repelling force between the permanent magnet and the armature magnetic pole in addition to the reluctance torque, a large torque can be obtained as a whole, and the output density per volume of the electric motor can be increased.
With a permanent magnet electric motor in which a permanent magnet is built into this type of rotor, since the interlinkage magnetic flux of the permanent magnet is generated constantly at a given strength, the induced voltage generated by the permanent magnet will increase in proportion to the rotating speed. Thus, when performing variable speed operation from a low speed to a high speed, the induced voltage (counter electromotive voltage) generated by the permanent magnet will become extremely high in a high-speed rotation. When the induced voltage generated by the permanent magnet is applied to the electronic parts of an inverter and becomes a withstand voltage or higher, the electronic parts will break down. Thus, considered may be a design where the flux content of the permanent magnet is reduced so that it will be the withstand voltage or less, but in the foregoing case, the output and efficiency of the permanent magnet electric motor will deteriorate in a low speed area.
Thus, proposed is technology of disposing, within the rotor, a permanent magnet of low coercive force of a level in which the magnetic flux density is irreversibly changed by the magnetic field created with a d-axis current of a stator winding (hereinafter referred to as the “variable magnetic force magnet”) and a permanent magnet of high coercive force having coercive force that is twice or more than that of the variable magnetic force magnet (hereinafter referred to as the “fixed magnetic force magnet”), and adjusting the total amount of interlinkage magnetic flux so that the total interlinkage magnetic flux generated by the variable magnetic force magnet and the fixed magnetic force magnet will decrease in a high revolution area where the power-supply voltage becomes a maximum voltage or greater (refer to Patent Document 2 and Patent Document 3).
Note that, since the flux content of the permanent magnet is decided based on the product of the coercive force and the thickness in the magnetization direction, when actually mounting the variable magnetic force magnet and the fixed magnetic force magnet in the rotor core, a permanent magnet in which the product of the coercive force and the thickness in the magnetization direction is small is used as the variable magnetic force magnet, and a permanent magnet in which the product of the coercive force and the thickness in the magnetization direction is large is used as the fixed magnetic force magnet. Moreover, generally speaking, an alnico magnet, a samarium-cobalt magnet (Sm—Co magnet) or a ferrite magnet is used as the variable magnetic force magnet, and a neodymium magnet (NdFeB magnet) is used as the fixed magnetic force magnet.
In this type of permanent magnet electric motor, when magnetizing a variable magnetic force magnet that was once demagnetized in a high revolution area, a phenomenon occurs where the magnetic field of the fixed magnetic force magnet disposed in the vicinity of the variable magnetic force magnet obstructs with the magnetization magnetic field that is created by a d-axis current, and the d-axis current (magnetization current) for the magnetization increases by that much. In order to deal with this kind of phenomenon, the present inventors and others proposed a permanent magnet electric motor capable of inhibiting the increase of the d-axis current during magnetization by disposing a short circuited coil in the vicinity of a fixed magnetic force magnet, generating an induced current in the short circuited coil based on a magnetic field generated by the d-axis current penetrating the short circuited coil, and negating the magnetic field that is generated in the fixed magnetic force magnet by using the foregoing induced current (Japanese Patent Application No. 2008-162203).
Patent Document 1: Japanese Patent Application Publication No. 2005-51897
Patent Document 2: Japanese Patent Application Publication No. 2006-280195
Patent Document 3: Japanese Patent Application Publication No. 2008-48514
Meanwhile, with a permanent magnet electric motor that is demanded of a compact size and high output, a large current and excitation magnetic force are required for obtaining high torque and high output, and since the armature field of return action is applied to the permanent magnet, there is a problem in that the permanent magnet becomes demagnetized. In addition, with a conventional reluctance-type electric motor, as shown in FIG. 19 and FIG. 20, a stepped skew of displacing and binding the block-shaped rotor laminated cores 2a, 2b and the permanent magnets 30a, 30b built therein in the circumferential direction is used to reduce the torque ripples, vibrations and noise. However, since the rotor cores 2a, 2b and the end faces of the permanent magnets 30a, 30b are in contact on the divided skew face S, the diamagnetic field generated by the armature reaction from the rotor cores 2a, 2b is applied to the end face and corners of the permanent magnets 30a, 30b, and, since the anti-demagnetization properties are weak, this causes the permanent magnet to become demagnetized.
Moreover, with a conventional variable magnetic force magnet-type electric motor, a stepped skewing as shown in FIG. 21 is performed by dividing the rotor core in order to similarly alleviate the torque ripples, vibrations and noise. In this kind of electric motor, when magnetizing a permanent magnet having a variable magnetic force configuring the magnetic pole of the rotor based on the magnetic field that is created by the armature winding, since the positions of the variable magnetic force magnets are different between the divided core parts, the magnetization directions of the variable magnetic force magnets are different in the axial direction (refer to FIG. 22), and it is therefore difficult to magnetize the variable magnetic force magnets, sufficient magnetization cannot be performed, and the magnetization current will increase.
In addition, in a variable magnetic force magnet-type electric motor, as shown in FIG. 23, since the rotor is internally provided with a conductive short circuited coil 8 to which flows a short-circuit current based on the magnetic flux that is generated during magnetization upon magnetizing the variable magnetic force magnet 3, it is necessary to bend the short circuited coil 8 at the divided skew face, and the insertion and assembly of the short circuited coil 8 are difficult, and the manufacturability of the rotor is extremely inferior. Particularly, certain variable magnetic force magnet-type electric motors have fixed magnetic force magnets 4a, 4b disposed on either end of the variable magnetic force magnets 3a, 3b, and, since the short circuited coil 8 is disposed to surround the variable magnetic force magnets 3a, 3b and the adjacent fixed magnetic force magnets 4a, 4b in each of the divided cores, if the short circuited coil 8 is bent at the divided skew face, difficult operations will be required for mounting the short circuited coil 8 in the core.